In the digital CS (communication satellite) broadcasting and digital BS (broadcasting satellite) broadcasting, AV signals of a plurality of programs (so-called broadcast program) are encoded by the MPEG2 method and subjected to time-division multiplexing to provide an MPEG2 transport stream (will be referred to as “MPEG2-TS” hereunder) which is sent as a broadcast signal.
Referring now to FIGS. 1A and 1B, there is explained an MPEG2 partial TS multiplexed in an MPEG2-TS. Some of receivers such as a set top box (STB), which is adapted to receive such a broadcast signal, extracts, from a received MPEG2-TS as shown in FIG. 1A, an MPEG2 partial transport stream being data corresponding to an arbitrary program as shown in FIG. 1B, and sends it to another electronic device via an IEEE (Institute of Electrical and Electronic Engineers) 1394 serial data bus.
Note that an MPEG-TS has inserted therein repeatedly in a cycle of less than 100 msec a time stamp called “PCR (program clock reference)” for a decoder. A PCR includes a program_clock_reference_base and a program_clock_reference_extension, which are recorded in an adaptation field of an MPEG2-TS packet. The time stamp PCR indicates a time at which the last bit of the program_clock_reference_base has arrived. It is specified that the allowable error of the PCR arrival time should be ±500 nanoseconds, and that the PCR-defined system clock should be 27 MHz with a resolution of 300 ppm.
For transmission of an MPEG2 partial TS via the IEEE 1394 serial data bus, a method called “isochronous transfer” is used.
In the isochronous transfer, each of IEEE 1394 interfaces at the sending and receiving sides has a sync clock (cycle timer), and a time stamp generated based on the clock is appended to an MPEG2-TS packet to correct the timing of reception at the receiving side.
FIG. 2 shows a format of a source packet consisting of a source packet header having a time stamp stored therein, a sync byte, and MEG2-TS packets. The time stamp is represented by a 13-bit cycle_count indicating a cycle timer and a 12-bit cycle_offset indicating a predetermined delay time, added to the cycle count.
FIGS. 3A to 3D show a time at which an MPEG2 partial TS is sent on the IEEE 1394 serial data bus.
As shown in FIG. 3A, an MGEP2 partial TS corresponding to a program extracted from the MPEG2-TS consists of MPEG2-TS packets laid at irregular intervals. A source packet header having stored therein a time stamp being a time counted by a cycle timer included in the IEEE 1394 interface at the sending side and to which a predetermined delay time is added, is appended to the MPEG2-TS packet to produce a source packet.
As shown in FIG. 3B, the source packets are sent at regular intervals from the sending side at times specified as in IEEE 1394. The source packets thus sent will be caused by a delay of transmission on the IEEE 1394 serial data bus or the like to arrive at the receiving side at irregular times as shown in FIG. 3C.
At the receiving side, however, since the source packets are stored in a smoothing buffer once, and then taken out at times specified by the time stamps, the MPEG2 partial TS shown in FIG. 3A is restored as shown in FIG. 3D.
Conventionally, an MPEG2 partial TS stored in each source packet and sent on the IEEE 1394 serial data bus as in the above, is received and recorded in a recording medium such as a magnetic tape, hard disc or the like. Also, such an MPEG2 partial TS is read out from the recording medium and sent on the IEEE 1394 serial data bus.
In the conventional apparatus, to reserve the timing of the original MPEG2 partial TS packet, the MPEG2 partial TS packet as well as a source packet header of a source packet having stored the MPEG2 partial TS packet are recorded in the recording medium, or a time stamp based on a clock which oscillates synchronously with the source packet header, a clock which oscillates synchronously with a PCR included in the MPEG2 partial TS or a clock which oscillates uniquely, is appended to the MPEG2 partial TS packet and recorded in the recording medium.
In a personal computer having a hard disc or in an AV hard disc recorder, for example, which can read bit streams at an arbitrary rate for decoding, the bit streams are read from the recording medium correspondingly to the extent of filling of the bit streams in a buffer incorporated in the decoder and thus bit streams will not overflow or underflow the buffer. Therefore, the MPEG2 partial TS packet have only to be timed for reproduction. Rather, in case timing information is recorded in the MPEG2 partial TS packet, a mechanism is required which removes the timing information and extracts only the MPEG2 partial TS packet.
On the other hand, to read an MPEG2 partial TS packet having timing information recorded therein for transmission on the IEEE 1394 serial data bus, a mechanism is required which restores the timing of the MPEG2 partial TS packet.
Conventionally, to restore the timing of the MPEG2 partial TS packet, there are required a mechanism which measures and records a time at which each MPEG2 partial TS packet has arrived, and a mechanism which restores original timing based on time stamps having been appended to the MEG2 partial TS packet at the time of reading and transmission. Also, the recording medium has to have a capacity for recording the time stamps.
Further, in some of the apparatuses including the above mechanisms for recording timing and restoring the recorded timing, an internal clock measures a time at which packets arrive and restores the timing. In case recording and reproduction are repeated with such apparatuses connected to each other, PCR errors will possibly be cumulated due to a difference in accuracy between the internal clock and PCR.
Furthermore, in case an MPEG2 partial TS is sent by an isochronous transfer on the IEEE 1394 serial data bus, a sufficiently broader band than a transport rate at which MPEG2-TS packets multiplexed in an MPEG2 partial TS corresponding to one program is secured on the IEEE 1394 serial data bus. However, the mean transport rate at which the MPEG2 partial TS is sent may be narrower than the secured sufficiently broad band, and also such a bandwidth of the IEEE 1394 serial data bus is uselessly wider since the MPEG2 partial TS is not sent at any constant transport rate.